1. Field of the Invention
The present invention relates to a circuit board transmitting a high frequency signal. More specifically, the present invention relates to a circuit board used for optical pickup to perform recording/regeneration or regeneration for a magnetic recording medium such as a magneto-optical disc.
2. Description of the Background Art
A laser diode for optical pickup is usually used to read information of DVD (Digital Versatile Disc), MD (Mini Disc) or the like. In the optical pickup for regeneration, a magneto-optical disc is irradiated with a light beam and reflected light thereof is received to regenerate recorded information utilizing a Kerr effect. During regeneration of the recorded information of the magneto-optical disc, if there is optical feedback from a side of the magneto-optical disc to an end surface of the laser diode, modulation may occur and accurate regeneration may not be possible. Therefore, to cancel the effect of optical feedback to the end surface of the laser diode, a technique has been adopted for superposing a high frequency current of several hundred MHz on a drive signal (current) driving the laser diode using a high frequency superposition circuit including a high frequency oscillator.
Since the high frequency current is superposed using the high frequency superposition circuit, however, unnecessary radiation noise of a high frequency component is generated on the periphery.
Various suggestions have been made conventionally to suppress the radiation noise. In Japanese Patent Laying-Open No. 5-120928, for example, a pair of transmission paths consisting of a feed path and a feedback path for transmitting a signal on a circuit board are formed to meander to each other, and one circuit conductor and the other circuit conductor form a region surrounded with the one circuit conductor and the other circuit conductor between two points of intersection with each other when seen from a direction perpendicular to a plane direction of the circuit conductor to obtain a twisted pair effect. A technique for suppressing the unnecessary radiation noise by obtaining the twisted pair effect is adopted. Another technique for obtaining the twisted pair effect is to provide a through hole in an insulation layer of a substrate and twist the transmission paths three-dimensionally via the through hole with alternate transition from one circuit conductor to the other circuit conductor of the insulation layer.
To obtain the twisted pair effect sufficiently, however, a sufficiently large region surrounded with one circuit conductor and the other circuit conductor between two points of intersection of the feed path and the feedback path of the signal must be ensured. In order to form the large region, a lateral width of the meander of at least three times larger than an interconnection width of the feed or feedback path must be ensured.
In the optical pickup for recording, on the other hand, in place of receiving the reflected light to read the recorded information, the magneto-optical disc is irradiated with a strong light beam to increase a temperature on the disc to record a signal. Therefore, the high frequency current is not superposed as cancellation of the effect of optical feedback as for the regeneration is not necessary. During the recording, as irradiation with a light beam stronger than that for regeneration is needed, a current of several times larger than that for regeneration must be fed to the laser diode. Therefore, a certain degree of the interconnection width is needed to allow flowing of the current of several times larger than that for regeneration. When the interconnection width is made wider, however, tight meandering becomes difficult, and thus it becomes difficult to generate a sufficient twisted pair effect.
In addition, when the twisted pair effect is obtained using the through hole, though the twisted pair effect can be expected according to a number of the through holes, a direct current resistance component increases when the number of the through holes is increased, which inhibits ensuring of a current for recording.